The molecular identities of many essential components of the hair cell stereocilia in the inner ear are still unknown, precluding our understanding of the precise mechanisms of the mechanotransduction (MET) of sound. To this end, studies of the gene/protein determinants of Usher Syndrome (USH), a neurosensory disorder affecting both hearing and vision in humans, have been fruitful in elucidating the molecular components of transduction machinery in the inner ear. The long-term goal of our research is to understand fully how USH proteins are involved in the biological processes of the inner ear as a means of developing new strategies to prevent and treat this dual neurosensory disorder. The objective of this particular application is to determine the role of a newly identifid USH protein, CIB2, in stereocilia bundle formation, Ca2+ homeostasis and MET, as well as the mechanisms underlying hearing impairment caused by the loss of CIB2 function. The CIB2 protein is encoded by CIB2, the causative gene for Usher syndrome type 1 and non-syndromic deafness in 58 human families. CIB2 is localized at the upper part of stereocilia of both inner and outer hair cells in the mouse inner ear. Based on strong preliminary data, the central hypothesis of this proposal is that CIB2 is a Ca2+-buffering protein required for the maintenance of Ca2+ homeostasis in the mechanosensory stereocilia of the inner ear hair cells. Loss of CIB2 function, then, will affect both the structure and mechanosensitivity of stereocilia bundles, resulting in hearing loss. The rationale for the proposed research is that once the function of CIB2 in the inner ear is known, this knowledge will improve our understanding of auditory MET and Ca2+ homeostasis processes at the molecular level, which will ultimately aid in developing therapeutic agents for preventing or reversing hearing loss. Thus, the proposed research is relevant to that part of NIH's mission that pertains to developing fundamental knowledge that will potentially help to reduce the burdens of human disability. This hypothesis will be tested by pursuing three specific aims: (1) elucidate the mechanism of hearing loss in Cib2F91S mice, (2) elucidate the role of CIB2 in the development and function of stereocilia bundles, and (3) elucidate potential interactions between CIB2 and known USH proteins present in the inner ear hair cell stereocilia. Our experimental approach is to define the consequences of loss of CIB2 on inner ear morphology, the MET current, adaptation, Ca2+ concentration in the stereocilia, cytoskeletal and ultra-structural alterations. Our studies will employ contemporary genetic, molecular, cellular, histochemical and physiological techniques. The approach is innovative, because CIB2 is a novel USH protein, therefore, its role in the hearing and vision processes has never been investigated. The proposed research is significant, first, because it is expected to provide mechanistic understanding of how CIB2 is involved in the structural development and function of inner ear hair cell stereocilia. Secondly, the proposed study will likely uncover the mechanisms and physiological significance of Ca2+ buffering in stereocilia, which is critically important in auditory MET.